Failover system and method

ABSTRACT

One aspect of the present invention provides a system for failover comprising at least one client selectively connectable to one of at least two interconnected server via a network connection. In a normal state, one of the servers is designated a primary server when connected to the client and a remainder of the servers are designated as backup servers when not connected to the client. The at least one client is configured to send messages to the primary server. The servers are configured to process the messages using at least one service that is identical in each of the servers. The services are unaware of whether a server respective to the service is operating as the primary server or the backup server. The servers are further configured to maintain a library, or the like, that indicates whether a server is the primary server or a server is the backup server. The services within each server are to make external calls via its respective library. The library in the primary server is configured to complete the external calls and return results of the external calls to the service in the primary server and to forward results of the external calls to the service in the backup server. The library in the secondary server does not make external calls but simply forwards the results of the external calls, as received from the primary server, to the service in the secondary server when requested to do so by the service in the secondary server.

FIELD OF THE INVENTION

The present invention relates to computer and network architecture andmore particularly relates to a failover system and method.

BACKGROUND OF THE INVENTION

Society is increasingly relying on computers and networks to interactand conduct business. To achieve a high level of availability demandedin critical systems, unplanned downtime caused by software and hardwaredefects should be minimized.

The financial services industry is but one example of an industry thatdemands highly available systems. Indeed, a large number of dataprocessing activities in today's financial industry are supported bycomputer systems. Particularly interesting are the so-called “real-time”and “near real-time” On-Line Transaction Processing (OLTP) applications,which typically process large numbers of business transactions over aprolonged period, with high speed and low latency. These applicationsgenerally exhibit the following characteristics: (1) complex and highspeed data processing, (2) reliable non-volatile data storage, and (3)high level of availability, i.e. the ability to support the services ona substantially uninterrupted basis. When implemented, however, existingapplications tend to tradeoff between these performance requirements,since, due to their contradictory effects on the system behavior, nodesign can completely satisfy all of three characteristicssimultaneously, as outlined in greater detail below.

First, complex data processing refers to the ability to perform, in atimely fashion, a large number of computations, databaseretrievals/updates, etc. This can be implemented through parallelprocessing, where multiple units of work are executed simultaneously onthe same physical machine or on a distributed network. In some systems,the outcome of each transaction depends on the outcomes of previouslycompleted transactions. The parallel aspects of such systems are,inherently, non-deterministic: due to race conditions, operating systemscheduling tasks, or variable network delays, the sequence of messageand thread execution can not be predicted, nor can they be processed inparallel simply by passing copies of input message to a duplicatesystem. Non-deterministic systems have non-identical output, so they arenot run in parallel on two different computing machines, with theintention of having one substitute for the other in case of failure.

Second, reliable non-volatile data storage refers to the ability topersistently store the processed data, even if a number of the system'ssoftware or hardware components experience unexpected failure. This canusually be implemented by using Atomic, Consistent, Isolated, andDurable (“ACID”) transactions when accessing or modifying the shareddata. ACID transactions can ensure the data integrity and persistence assoon as a unit of work is completed. Every committed ACID transaction iswritten into the non-volatile computer memory (hard-disk), which helpsensure the data durability, but it is very costly in terms ofperformance and typically slows down the whole system.

Third, highly available systems attempt to ensure that percentage ofavailability of a given computer system is as close as possible to 100%of the time. Such availability can be implemented through redundantsoftware and/or hardware, which takes over the functionality in case acomponent failure is detected. In order to succeed, the failoverreplicates not only the data, but also the process state. As will beappreciated by those of skill in the art, state replication can beparticularly challenging in non-deterministic systems (i.e. systemswhere computational processing of the same set of events can have morethan one result depending on the order in which those events areprocessed).

Highly available software applications are usually deployed on redundantenvironments, to reduce and/or eliminate the single point of failurethat is commonly associated with the underlying hardware. Two commonapproaches are known as hot failover and warm failover. Hot failoverrefers to simultaneously processing the same input in multiple systems,essentially providing complete redundancy in the event of a failure inone of those systems. Warm failover refers to replicating the state ofthe application (i.e. the data) in backup systems, without processingthat data in the backup systems, but having applications capable ofprocessing that data loaded and standing by in the event of failure of aprimary system. Cold failover, which is not considered by many to be aform of high availability, refers to simply powering-up a backup systemand preparing that backup system to assume processing responsibilitiesfrom the primary system.

In hot failover configurations, two instances of the application aresimultaneously running on two different hardware facilities, processingcopies of the same input. If one of them experiences a critical failure,a supplemental synchronization system can ensure that the other one willcontinue to support the workload. In the warm failover configurations,one of the systems, designated primary, is running the application; incase of failure, the second system, designated backup, which is waitingin a standby state, will “wake up”, take over, and resume thefunctionality.

Prior art hot failover approaches have at least two disadvantages.First, supplemental software has to run in order to keep the two systemssynchronized. In the case of non-deterministic systems, thissynchronization effort can lead to an unacceptable (or otherwiseundesirable) decrease in performance and complexity where the order ofarrival of events must be guaranteed to be identical. Also, prior artconcurrent systems used in such applications typically allow multiplethreads to execute simultaneously, so they are inherentlynon-deterministic. Also non-deterministic are the systems with serversand geographically distributed clients, where the variable network delaydelivers the messages to the server in an unpredictable sequence.

Warm failover can be used to overcome certain problems with hotfailover. Warm failover can be another way to implement failover ofnon-deterministic systems, by replicating the system data to aredundant, backup system, and then restoring the applicationfunctionality to the secondary system. This approach has its drawbacksin the time required to recover the data to a consistent state, then tobring the application to a functional state, and lastly, to return theapplication to the point in processing where it left off. This processnormally takes hours, requires manual intervention, and cannot generallyrecover the in-flight transactions.

A number of patents attempt to address at least some of the foregoingproblems. U.S. Pat. No. 5,305,200 proposes what is essentially anon-repudiation mechanism for communications in a negotiated tradingscenario between a buyer/seller and a dealer (market maker). Redundancyis provided to ensure the non-repudiation mechanism works in the eventof a failure. It does not address the fail-over of an on-linetransactional application in a non-deterministic environment. In simpleterms, U.S. Pat. No. 5,305,200 is directed to providing an unequivocalanswer to the question: “Was the order sent, or not?” after experiencinga network failure.

U.S. Pat. No. 5,381,545 proposes a technique for backing up stored data(in a database) while updates are still being made to the data. U.S.Pat. No. 5,987,432 addresses a fault-tolerant market data ticker plantsystem for assembling world-wide financial market data for regionaldistribution. This is a deterministic environment, and the solutionfocuses on providing an uninterrupted one-way flow of data to theconsumers. U.S. Pat. No. 6,154,847 provides an improved method ofrolling back transactions by combining a transaction log on traditionalnon-volatile storage with a transaction list in volatile storage. U.S.Pat. No. 6,199,055 proposes a method of conducting distributedtransactions between a system and a portable processor across anunsecured communications link. U.S. Pat. No. 6,199,055 deals withauthentication, ensuring complete transactions with remote devices, andwith resetting the remote devices in the event of a failure. In general,the foregoing do not address the fail-over of an on-line transactionalapplication in a non-deterministic environment.

U.S. Pat. No. 6,202,149 proposes a method and apparatus forautomatically redistributing tasks to reduce the effect of a computeroutage. The apparatus includes at least one redundancy group comprisedof one or more computing systems, which in turn are themselves comprisedof one or more computing partitions. The partition includes copies of adatabase schema that are replicated at each computing system partition.The redundancy group monitors the status of the computing systems andthe computing system partitions, and assigns a task to the computingsystems based on the monitored status of the computing systems. Oneproblem with U.S. Pat. No. 6,202,149 is that it does not teach how torecover workflow when a backup system assumes responsibility forprocessing transactions, but instead directs itself to the replicationof an entire database which can be inefficient and/or slow. Further,such replication can cause important transactional information to belost in flight, particularly during a failure of the primary system orthe network interconnecting the primary and backup system, therebyleading to an inconsistent state between the primary and backup. Ingeneral, U.S. Pat. No. 6,202,149 lacks certain features that are desiredin the processing of on-line transactions and the like, and inparticular lacks features needed to failover non-deterministic systems.

U.S. Pat No. 6,308,287 proposes a method of detecting a failure of acomponent transaction, backing it out, storing a failure indicatorreliably so that it is recoverable after a system failure, and thenmaking this failure indicator available to a further transaction. Itdoes not address the fail-over of a transactional application in anon-deterministic environment. U.S. Pat. No. 6,574,750 proposes a systemof distributed, replicated objects, where the objects arenon-deterministic. It proposes a method of guaranteeing consistency andlimiting roll-back in the event of the failure of a replicated object. Amethod is described where an object receives an incoming client requestand compares the request ID to a log of all requests previouslyprocessed by replicas of the object. If a match is found, then theassociated response is returned to the client. However, this method inisolation is not sufficient to solve the various problems in the priorart.

Another problem is that the method of U.S. Pat. No. 6,575,750 assumes asynchronous invocation chain, which is inappropriate forhigh-performance On-Line Transaction Processing (“OLTP”) applications.With a synchronous invocation the client waits for either a reply or atime-out before continuing. The invoked object in turn may become aclient of another object, propagating the synchronous call chain. Theresult can be an extensive synchronous operation, blocking the clientprocessing and requiring long time-outs to be configured in theoriginating client.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a system for failovercomprising at least one client selectively connectable to one of atleast two interconnected servers via a network connection. In a normalstate, one of the servers is designated a primary server when connectedto the client and a remainder of the servers are designated as backupservers when not connected to the client. The at least one client isconfigured to send messages to the primary server. The servers areconfigured to process the messages using at least one service that isidentical in each of the servers. The services are unaware of whether aserver respective to the service is operating as the primary server orthe backup server. The servers are further configured to maintain alibrary or other distinct set(s) of usable code that performs a varietyof tasks, including indicating whether a server is the primary server ora server is the backup server. The services within each server are tomake external calls to its respective library. The library in theprimary server is configured to complete the external calls and returnresults of the external calls to the service in the primary server andto forward results of the external calls to the service in the backupserver. The library in the secondary server does not make external callsbut simply forwards the results of the external calls, as received fromthe primary server, to the service in the secondary server whenrequested to do so by the service in the secondary server.

The library can be implemented as one or more distinct sets of usablecode.

The servers can each be configured to maintain a shared resource wherethe services can store results of processing the messages. The sharedresource can be, and for performance reasons, are preferably, maintainedin random access memory of a respective server. However, it is notnecessary to maintain the shared resource in random access memory.

The external call can be, (as a non-limiting list of examples) a requestfor a time stamp, or call to another service provided on the sameserver, or a call to another service provided physically on a separatemachine.

The system can be part of an electronic trading system and the messagecan thus be an order to buy or sell a security. In this case theexternal call can be a request for a market feed quote for a value ofthe security. Where the system is an electronic trading system, the atleast one service can include one of an order placement service; anorder cancellation service; an order change service; an order matchingservice; a service to enter a previously-executed trade; or a service toenter a cross trade.

The service in the primary server can be configured to confirm to theclient that the message has been processed only if the backup serverconfirms that the results of the external calls were successfullyforwarded to the backup server.

The service in the primary server can be configured to confirm to theclient that the message has been processed regardless of whether thebackup server confirms that the results of the external calls weresuccessfully forwarded to the backup server. The primary server can deemthe backup server to have failed if the backup server does not confirmthat the results of the external calls were successfully forwarded tothe backup server within a predefined time period.

Another aspect of the invention provides a method for failover in asystem comprising:

-   at least one client selectively connectable to one of at least two    interconnected servers via a network connection; one of the servers    being designated a primary server when connected to the client and a    remainder of the servers being designated a backup server when not    connected to the client; the at least one client configured to send    messages to the primary server; the method comprising:-   configuring the servers to process the messages using at least one    service that is identical in each of the servers and is unaware of    whether a server respective to the service is operating as the    primary server or the backup server;-   configuring the servers to maintain a library that indicates whether    the server is the primary server or the server is the backup server;-   configuring the services to make external calls to its respective    the library; and,-   configuring the library in the primary server to complete the    external calls and return results of the external calls to the    service in the primary server and to forward results of the external    calls to the service in the backup server.

Another aspect of the invention provides a computer readable mediumstoring a set of programming instructions executable on one of at leasttwo interconnected servers via a network connection that are selectivelyconnectable to at least one client. One of the servers can be designateda primary server when connected to the client in which case a remainderof the servers are designated a backup server when not connected to theclient. The at least one client is configured to send messages to theprimary server. The programming instructions comprise:

-   instructions for configuring the servers to process the messages    using at least one service that is identical in each of the servers    and is unaware of whether a server respective to the service is    operating as the primary server or the backup server;-   instructions for configuring the servers to maintain a library that    indicates whether the server is the primary server or the server is    the backup server;-   instructions for configuring the services to make external calls to    its respective the library; and,-   instructions for configuring the library in the primary server to    complete the external calls and return results of the external calls    to the service in the primary server and to forward results of the    external calls to the service in the backup server.

BRIEF DESCRIPTION Of THE DRAWINGS

The invention will now be described by way of example only, and withreference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a system for failover inaccordance with an embodiment of the invention;

FIG. 2 is a schematic representation of the system in FIG. 1 asoperating in a normal state and including exemplary details of varioussoftware elements executing on the servers in the system;

FIG. 3 is a flowchart representing a method of operating a system forfailover in the normal state in accordance with another embodiment ofthe invention;

FIG. 4 shows the system of FIG. 2 during performance of the method inFIG. 3;

FIG. 5 shows the system of FIG. 2 during performance of the method inFIG. 3;

FIG. 6 shows the system of FIG. 2 during performance of the method inFIG. 3;

FIG. 7 shows the system of FIG. 2 during performance of the method inFIG. 3;

FIG. 8 shows the system of FIG. 2 during performance of the method inFIG. 3;

FIG. 9 shows the system of FIG. 2 during performance of the method inFIG. 3;

FIG. 10 shows the system of FIG. 2 during performance of the method inFIG. 3;

FIG. 11 shows the system of FIG. 2 during performance of the method inFIG. 3;

FIG. 12 shows the system of FIG. 2 during performance of the method inFIG. 3;

FIG. 13 shows the system of FIG. 2 during performance of the method inFIG. 3;

FIG. 14 shows the system of FIG. 2 during performance of the method inFIG. 3;

FIG. 15 is a flowchart representing a method for failover in accordancewith another embodiment of the invention;

FIG. 16 shows the system of FIG. 2 with one of the servers operating inthe primary-only state in accordance with another embodiment of theinvention;

FIG. 17 shows the system of FIG. 16 with the other server operating inthe primary-only state in accordance with another embodiment of theinvention;

FIG. 18 is a flowchart representing a method for operating one of theservers in the primary-only state in accordance with another embodimentof the invention; and,

FIG. 19 is a flowchart representing a method for failing over from thenormal state to the backup server operating in the primary-only state inaccordance with another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a system for failover is indicated generally at50. System 50 comprises a plurality of remote clients 54-1 and 54-2(generically referred to herein as “client 54” and collectively as“clients 54”. This nomenclature is used for other elements in theFigures). Clients 54 are connected to a network 58. Network 58 can beany type of computing network, such as the Internet, a local areanetwork, a wide area network or combinations thereof. In turn, network58 is connected to a first server 62-1 and a second server 62-2.Accordingly, clients 54 can each communicate with server 62-1 and server62-2 via network 58, as will be discussed in greater detail below.

Clients 54 each belong to individuals and/or entities that will usetheir respective client 54 to submit requests to server 62-2. Forconvenience, such individuals or entities are referred to herein astraders T, with trader T-1 using client 54-1, and trader T-2 usingclient 54-2. Each client 54 is typically a computing device such as apersonal computer having a keyboard and mouse (or other input devices),a monitor (or other output device) and a desktop-module connecting thekeyboard, mouse and monitor and housing one or more central processingunits, volatile memory (i.e. random access memory), non-volatile memory(i.e. hard disk devices) and network interfaces to allow the client 54to communicate over network 58. However, it is to be understood thatclient 54 can be any type of computing device, such as a personaldigital assistant, cell phone, laptop computer, email paging device etc.

Servers 62 can be any type of computing device operable to receive andprocess messages from clients 54, such as Sun Fire V480 running a UNIXoperating system, from Sun Microsystems, Inc. of Palo Alto Calif., andhaving four central processing units each operating at about 900megahertz and having about four gigabytes of random access memory and anon-volatile storage device such as a hard disc drive. Another type ofcomputing device suitable for servers 62 is a HP ProLiant BL25p serverfrom Hewlett-Packard Company, 800 South Taft, Loveland, Colo. 80537.However, it is to be emphasized that these particular servers are merelyexemplary, a vast array of other types of computing environments forservers 62-1 and 62-2 are within the scope of the invention. The type ofmessage being received and processed by server 62-1 is not particularlylimited, but in a present embodiment, server 62-1 operates an on-linetrading system, and is thus able to process messages that includerequests to purchase, sell, cancel etc. securities that can be tradedon-line. More particularly, server 62-1 is operable to maintain acentral matching engine (not shown), where requests are executed againsteach other, and against a central repository of orders to therebyprocess the trading of securities.

Server 62-2 typically has an identical (or at least substantiallyidentical) computing environment as server 62-1. As will be explainedfurther below, the computing environment including its hardware,operating system, applications, etc. is thus chosen to render server62-2 operable to substitute the functionality of server 62-1 in theevent of a failure of server 62-1.

System 50 also includes a replication link 78 that interconnects server62-1 and server 62-2. In a present embodiment, replication link 78itself includes a main link 82 and a failsafe link 86 to provide greaterrobustness in communications between server 62-1 and server 62-2.

Further details about the functions of primary server 62-1, backupserver 62-2 and replication link 78, and by extension the various typesof hardware that can be used to implement servers 62-1 and 62-2, willbecome apparent in the discussion below.

In FIG. 2, servers 62-1 and 62-2 are shown in greater detail. Also ofnote, the various connections in FIG. 2 are shown in dashed lines, todenote virtual connections between various elements in system 50, incontrast to the solid line connections in FIG. 1, which denote thephysical connections between various elements of system 50. Thus, suchconnections as shown in FIG. 2 are intended to denote system 50operating in a normal state whereby server 62-1 is designated as theprimary server and server 62-2 is designated the backup server, withprimary server 62-1 servicing requests from clients 54. Further detailsabout the normal state, and other states in which system 50 can operate,will be provided below.

Referring still to FIG. 2, server 62-1 and server 62-2, each include aplurality of software elements that execute on their respective hardwareenvironments to service requests from clients and provide failoverfunctionality.

Server 62-1 and server 62-2 each include a failover agent 90-1 and 90-2respectively. Failover agents 90 communicate with each other and areoperable to periodically test the integrity of link 78 and each other.In a present embodiment, in the normal state, failover agent 90-1 willperiodically deliver a keep-alive signal (e.g. “Are you alive?”) tofailover agent 90-2, to which failover agent 90-2 is expected toperiodically respond (e.g. “Yes I am”). Provided such requests areresponded to by failover agent 90-2, and provided primary server 62-1continues to operate normally, then system 50 will remain in the normalstate shown in FIG. 2. Thus, failover agent 90-1 is also operable tocommunicate with other software elements in server 62-1 to indicate thatthe normal state is in effect.

It should now be apparent that failover agents 90 are operable to makeuse of both main link 82 and failsafe link 86 that together compriselink 78, as appropriate or as otherwise desired. In this manner, system50 can remain in the normal state as long as at least one of main link82 and failsafe link 86 are operational.

Servers 62 each include one or more services that can receive andprocess various requests from one or more clients 54. The types ofservices are not particularly limited and can include any type ofservice, application, or process or the like for which failoverprotection is desired. In a present, and purely exemplary embodiment,where system 50 is an on-line trading system, servers 62 each include anorder placement service 94 and an order cancellation service 98. Orderplacement service 94, as the name implies, is configured to receiverequests from clients 54 for placing of either a sell order or a buyorder for a particular security. Order cancellation service 98, as thename implies, is configured to receive requests from clients 54 forcancelling sell or buy orders for a particular security, that werepreviously-placed using service 94, but before that particular order isactually fulfilled. Other types of services that could be implemented,as will now occur to those skilled in the art of electronic trading,include, without limitation, order matching, change order, enter atrade, or enter a cross. In a present embodiment services 94 and 98 aremulti-threaded, though this is not a requirement. (As used herein,multi-threading is not used in a limiting sense, and refers to variousforms of concurrent processing where multiple messages are beingprocessed simultaneously, which further contributes to thenon-deterministic nature of systems. Multi-threading can be implemented,for example, using multiple processes, or using multiple threads ofexecution with a single process.)

Servers 62 each also include a library 102 that is accessible to thecorresponding services 94 and 98 respective thereto. Each library 102includes a sequencer 106 and a cache 110. As will be explained ingreater detail below, sequencer 106 generates a sequence number inresponse to a request from a service 94 or 98 respective to library 102.Sequencer 106-2 is inactive in the normal state and such inactivity isrepresented in FIG. 2 by the hashing through the oval representingsequencer 106-2. (Hashing is used in other elements to denote whetherthat element is active or inactive in any given particular state.) Cache110 is a storage area for results of external function calls made bylibrary 102.

Each library 102 also includes a state register 114 that maintains thestate in which system 50 is currently operating, and which continuouslycommunicates with its respective failover agent 90 in order to verifythe state in which system 50 is currently operating. In FIG. 2, system50 is operating in the normal state and accordingly state register 114-1indicates that server 62-1 is currently designated as the primary serverwhile state register 114-2 indicates that server 62-2 is currentlydesignated as the backup server. However, as will be explained ingreater detail below, the state of system 50 can change depending on theoperational status of various components in system 50.

Servers 62 each also include an external resource agent 118 which isresponsible for making external calls to external resources on behalf ofservices 94 and 98, but which are made via library 102. Externalresources can include resources that are external to services 94 and 98but resident on each server 62, such as a time stamp from operatingsystem clock (not shown), and/or resources that are external to eachserver 62 altogether, such as, in the case of an electronic tradingsystem, a market feed (not shown) that maintains up-to-date informationof market prices for various securities which may be the subject of abuy order or a sell order that is placed via order placement service 94.Those skilled in the art will now appreciate that calls by services 94and 98 to such external resources contribute to the non-deterministicnature of system 50. In the normal state, only external resource agent118-1 is active, while external resource agent 118-2 is inactive. Theinactivity of external resource agent 118-2 is represented in FIG. 2 bythe hashing through the oval representing external resource agent 118-2.

Servers 62 each also maintain a shared resource 122 which maintainsresults of processing steps performed by services 94 and 98 and/ormaintains data that may need to be accessible by services 94 and 98. Forexample, in the electronic trading system of the present embodiment,shared resource 122 maintains and order book, which is simply a set ofrecords of orders placed by service 94. Thus order placement service 94may, for example, create a record in shared resource 122 of a buy order.Such a buy order may need to be accessed at a later time by ordercancellation service 98 in order to cancel that buy order and indicateas such in shared resource 122. Likewise, the buy order may need to beaccessed by a matching service (not shown) that also executes on servers62 in order to match that buy order, according to market rules, with anappropriate corresponding sell order, and update that buy order and thatsell order to indicate that a match has been effected and a trade is tobe consummated.

Servers 62 each also maintain a replication agent 126. In the normalstate, only replication agent 126-2 is active, while replication agent126-1 is inactive. The inactivity of replication agent 126-1 isrepresented in FIG. 2 by the hashing through the oval representingreplication agent 126-1. As will be explained in greater detail below,an active replication agent 126 communicates with the library 102 in thecounterpart server 62 to facilitate the mirroring of information fromthe primary server to the backup server.

Referring now to FIG. 3, a method for processing requests during thenormal state in accordance with another embodiment of the invention isindicated generally at 300. In order to assist in the explanation of themethod, it will be assumed that method 300 is operated using system 50in the normal state shown in FIG. 2. Furthermore, the followingdiscussion of method 300 will lead to further understanding of system 50and its various components. For convenience only, however, variousprocess steps of method 300 are indicated in FIG. 3 as occurring withincertain components of system 50. Such indications are not to beconstrued in a limiting sense. It is to be understood, however, thatsystem 50 and/or method 300 can be varied, and need not work asdiscussed herein in conjunction with each other, and the steps in method300 need not be performed in the order as shown. Such variations arewithin the scope of the present invention. Such variations also apply toother methods and system diagrams discussed herein.

Beginning first at step 310, a message is received from a client. Thetype of message is not particularly limited and is generallycomplementary to an expected type of input for one of the servicesexecuting on the servers. When performed on system 50, the message canthus be a buy order or a sell order that is intended as input for orderplacement service 94, or can be a cancel order that is intended as inputfor order cancellation service 98. For example, assume that an order tobuy is placed into a message from client 54-1 by trader T-1 and themessage is sent over network 58 to order placement service 94-1, where,in accordance with step 310, the message is received by order placementservice 94-1. This exemplary performance of step 310 is shown in FIG. 4,as a message M(O₁) is shown as originating from client 54-1 and receivedin server 62-1 at order placement service 94-1. Table I shows anexemplary format of order placement message M(O₁).

TABLE I Message M(O₁) Field Example Number Field Name Contents 1 TraderTrader T-1 2 Security Name ABC Co. 3 Transaction Buy Type 4 Quantity1,000 units

More particularly, Field 1 of Table I, named “Trader” identifies thatthe originating trader of message M(O₁) is Trader T-1. Field 2 of TableII, named “Security Name” identifies the name of the specific securitythat is the subject of the trade—in this example, “ABC Co.”. Field 3 ofTable I, named “Transaction Type” identifies whether the order is tobuy, sell, etc. the security identified in Field 2. in this example, theTransaction Type is “Buy”, indicating that this is an order to buy.Field 4 of Table I, named “Quantity” identifies the desired quantity ofthe security—in the present example, the Quantity is “1,000 units”,indicating that the intention is to Buy 1,000 units of ABC Co. Thoseskilled in the art will now recognize that the order in Table I is amarket order, in that the price of the order will be based on whateverthe current market price is for the Security in Field 2.

Having received the message at step 310, method 300 advances to step 315at which point the relevant service will make any calls for externaldata utilized to further process the message. Continuing with theexample, at step 315 order placement service 94-1 will make suchexternal calls to primary library 102.1. In this example it will beassumed that such calls are for:

-   -   i) a time-stamp to assign to the order in message M(O₁)        identifying the time at which the order was received and,    -   ii) a current market price for the security identified in the        order in message M(O₁).

Performance of step 315 is represented in FIG. 5 as a dotted linerepresenting a call from order placement service 94-1 to primary library102-1 is indicated at 130.

Next, at step 320, primary library 102-1 will make the calls. Primarylibrary 102-1 will consult with failover agent 114-1 and confirm thatserver 62-1 is designated the primary server and that system 50 is inthe normal state. After so confirming, primary library 102-1 willrespond to calls made by service 94-1 by:

-   -   i) making an external call to external resource agent 118-1 in        order to obtain a time-stamp;    -   ii) making a further external call to external resource agent        118-1 in order to obtain the current market price.

Thus, at step 325, external resource agent 118-1 will make externalcalls to the operating system clock (not shown) and the market feed (notshown) to obtain a time-stamp and the current market price,respectively.

Performance of steps 320 and 325 are represented in FIG. 6 as dottedlines representing calls for a time stamp via external resource agent118-1 and a market price via external resource agent 118-1 are indicatedat 132 and 134, respectively.

Those skilled in the art will now recognize that external calls 132 and134, in particular, render system 50 non-deterministic in nature andtherefore present unique challenges in providing a failover system that,in the event of failover, addresses the non-deterministic nature of thesystem during the recovery such that the recovery is transparent totraders T. (By way of further explanation, assume that system 50 wasaltered so that both servers 62 made external calls for each message.Yet, for any given message M, the exact moment when a call is made for atime stamp is critical in order to ensure market fairness, and it ishighly unlikely that both servers 62 would make a call for a time stampfor the same message at the same time, and therefore each server 62could assign a different time priority for the same message M, resultingin differing outcomes of the same machine process. Likewise, for anygiven message M the exact moment when a call is made for a market priceis also critical in order to ensure market fairness, and it is highlyunlikely that both servers 62 would make a call for a market price forthe same message at the same time, and therefore each server 62 couldhave a different market price for the same message M. During a failover,each server 62 would not have consistent business data and the failoverwould be meaningless.) From reading further, those skilled in the artwill come to recognize how such challenges are addressed, as well asrecognizing other aspects of the invention.

At step 330, the results of external calls 132 and 134 are returned toprimary library 102-1. At step 335, the results of all calls 132 and 134are stored in cache 110-1 and returned to service 94-1.

Continuing with the example, it will be assumed that the result of call132 is the time-stamp 12:00 PM, Jan. 5, 2000; and it will be assumedthat the result of call 134 is the market price of $2.00. The storage ofthese results in cache 110-1 is represented in Table II and in FIG. 7.

TABLE II Exemplary Contents of Cache 110-1 after Step 335 Record FieldExample Number Number Field Name Contents 1 1 Message M(O₁) 1 2 TimeStamp 12:00 PM, Jan. 5, 2000 1 3 Market Price $2.00

At step 340, the call results are received by the service. Continuingwith the present example, the call results stored in Table II will bereturned to service 94-1, which is also represented in FIG. 7.

Next, at step 345, the service will make a request for shared resources.In the present example the request is made by service 94-1 to library102-1. In turn, at step 350, library 102-1 will issue an instruction toshared resource 122-1 to “lock” it and thereby prevent any otherservice, (e.g. service 98-1, or another thread within service 94-1),from accessing shared resource 122-1. (As will be described in greaterdetail below, if shared resources 122-1 is already locked, then method300 will pause at step 345 until shared resources 122-1 becomesunlocked). Performance of steps 345 and 350 are represented in FIG. 8 asa dotted line representing a request for shared resources indicated at140. The locking of shared resources 122-1 is represented by a padlock138.

Next, at step 355, a shared resource sequence number is returned. Thisstep can be performed by library 102-1 utilizing sequencer 106-1 togenerate a sequence number associated with message M(O₁). Continuingwith the example, it will be assumed that a sequence number of “one” isgenerated. The storage of these results in cache 110-1 is represented inTable III and FIG. 8. Note that Table III is an update of Table II.

TABLE III Exemplary Contents of Cache 110-1 after Step 355 Record FieldExample Number Number Field Name Contents 1 1 Message M(O₁) 1 2 TimeStamp 12:00 PM, Jan. 5, 2000 1 3 Market Price $2.00 1 4 Sequence 1Number

Next, at step 360, replication is requested. Step 360 in the presentexample is performed by service 94-1, which sends an instruction tolibrary 102-1 to perform replication. At step 365, replication of themessage, call results and sequence number is initiated. In the presentexample, the contents of Table III is replicated by library 102-1. Step365 will be discussed further later below.

At step 370, the message is processed using the call results and thelocked shared resources. In the present example step 370 is performed byservice 94-1, which uses the contents of Table III and performsprocessing steps associated with service 94-1 in order to generateresults from Table III. Since service 94-1 is an order placementservice, and message M(O₁) represents a buy order, then at step 370service 94-1 will generate a buy order that will be recorded in sharedresource 122-1 for subsequent matching with a sell order against a sellorder from, for example, trader T-2, or other trade processing such ascancellation of the order using service 98-1.

For purposes of the present example, it will be assumed that there areno orders in shared resources 122-1 against which message M(O₁) can bematched, and thus the results of step 370 will be to simply generate acomplete record of the details of the buy order associated with messageM(O₁). Table IV shows exemplary results of the performance of step 370.

TABLE IV Exemplary Results of performance of step 370 Record FieldExample Number Number Field Name Contents 1 1 Time Stamp 12:00 PM, Jan.5, 2000 1 2 Market Price $2.00 1 3 Sequence 1 Number 1 4 Trader TraderT-1 1 5 Security Name ABC Co. 1 6 Transaction Buy Type

Next, at step 375, the results of the performance of step 370 arewritten to the shared resources, and then the shared resources areunlocked. The generation of Table IV by service 94-1 at step 370, andthe storage of those results in shared resources 122-1 at step 375 isrepresented in FIG. 9.

Next, at step 380, the service confirms that the results have beenwritten at step 375, and a confirmation that replication has beenperformed at step 400. In the current example, at step 380, service 94-1will wait for a confirmation from shared resources 122-1 that the TableIV was written to shared resources 122-1. Likewise, at step 380, service94-1 will wait for confirmation, from step 400, that the replicationinitiated at step 365 has been completed. Steps 365 and 400 will beexplained in greater detail below.

In an alternative embodiment, step 380 need not actually wait for theconfirmation from step 400 before proceeding on to step 390. Howeverstep 380 would still expect to eventually receive such confirmation fromstep 400, and, if such confirmation was not forthcoming, then step 380would assume that server 62-2 had failed, in which event server 62-1would begin performing method 600 as explained later below. Thoseskilled in the art will now recognize that this is an asynchronous modeof operation and may be preferred in certain circumstances where speedis preferred over confirmation of the status of server 62-2.)

Next, at step 390, confirmation is returned to client. In the currentexample, at step 390 service 94-1 will send a confirmation message toclient 54-1 that message M(O₁) has been processed as requested by traderT-1.

It is to be reiterated that step 390 of method 300 (i.e. operationduring the normal state) is not completed until step 380, which in turnis not completed until the replication initiated at step 365 has beencompleted. Returning now to step 365, the message, call results andshared resource sequence numbers are replicated. In the present examplestep 365 is performed by library 102-1 responsive to the request fromservice 94-1 at step 360. Thus, library 102-1 will bundle the contentsof Table III and deliver it to replication agent 126-2.

The performance of Steps 365, 370, 375, 395, 400 and 390 are representedin FIG. 10. (FIG. 10 builds on the representation of performance ofsteps 370 and 375 in FIG. 9). Step 365, the delivery of Table III fromcache 110-1 of library 102-1 to replication agent 126-2 is representedby the line indicated at 142. Steps 370 and 375 are represented in FIG.10 as earlier discussed in relation to FIG. 9. Step 395, the queuing ofthe message, call results, and shared resource sequence number isrepresented by the oval marked as Table III appearing inside replicationagent 126-2. Step 400, the returning of confirmation of replication fromreplication agent 126-2 to service 94-1 (Carried via library 102-1), isrepresented by the line indicated at 144. Step 390, the returning ofconfirmation from service 94-1 to client 54-1, is represented by thedotted line indicated at 146.

The foregoing substantially completes the description of the processingof one message by primary server 62-1 during operation in the normalstate. It should now be understood that primary server 62-1 can processmultiple messages, either in series and/or substantially in parallelaccording to the above description of steps 310 through 400. Forexample, while service 94-1 is handling one message M, likewise service98-1 can also be processing another message M substantially as describedabove, with library 102-1 interacting with both services 94-1, 98-1.Additionally, while one thread of service 94-1 is handling one messageM, another thread of service 94-1 can also be processing another messageM substantially as described above, with library 102-1 interacting withboth threads of the service. Step 350 ensures that shared resource 122-1are locked to avoid contention between services 94-1 and 98-1 (orthreads thereof), to ensure that only one of those services can interactwith shared resource 122-1 at a time. (Note that “interact” can includeany type of function, including without limitation reading, writing, anddeleting). As an example of contention that needs to be avoided, ordercancellation service 98-1 would read from and write to shared resource122-1 while it is locked in order to cancel a given order, which wouldprevent a matching service (not shown) from matching with an order thatis being cancelled.

By the same token, step 355 utilizes sequencer 106-1 to generate uniquesequence numbers for each message M, and regardless of which service94-1 or 98-1 (or thread thereof) is handling the message M. Thus, theremay be times when a particular service 94-1 or 98-1 (or thread thereof)makes a request for shared resources 122-1 at step 345 while sharedresources 122-1 is locked, and therefore that particular service (orthread thereof) will pause at step 345 until shared resources 122-1 isunlocked before continuing onwards from step 345.

Having described the processing of messages by primary server 62-1during operation in the normal state, discussion of method 300 will nowturn to performance of steps 405 and onwards and the processing ofmessages by secondary server 62-2.

Referring again to FIG. 3, at step 405, messages, call results andsequence numbers are dispatched according to the shared resourcesequence number. Continuing with the example above, at this pointmessage M(O₁) (i.e. the contents of Field 1 of Record 1 from Table III)will be dispatched to service 94-2, while the call results (i.e. thecontents of Fields 2 and 3 of Record 1 from Table III) and sequencenumber (i.e. the contents of Field 4 of Record 1 from Table III) will bedispatched to secondary library 102-2.

Thus, at step 310S service 94-2 will receive message M(O₁) fromreplication agent 126-2 in much the same way that, at step 310, service94-1 received message M(O₁) from client 54-1. From the perspective ofservice 94-2, message M(O₁) has been received from a client. At thispoint it will now become apparent that service 94-2 is substantiallyidentical in all ways to service 94-1. (Likewise service 98-2 issubstantially identical to service 98-1). Service 94-2 will operate inserver 62-2 in substantially the same manner that service 94-1 operatesin server 62-1. In other words, steps 310S, 315S, 340S, 345S, 360S,370S, 380S and 390S are performed by service 94-2 in the same manner assteps 310, 315, 340, 345, 360, 370, 380 and 390 are performed by service94-1 in server 62-1. Neither service 94-1, nor service 94-2 are aware ofwhether the particular server they are operating within are designatedas primary server or backup server. This presents one of the manyadvantages of the present invention, as services can be developed oncefor two (or more) servers, without having to develop one set of servicesfor a server designated as a primary server and one set of services fora server designated as a backup server.

However each library 102, in consultation with its respective failoveragent 90 and state register 114, is aware of whether its respectiveserver 62 is designated as a primary server or as a backup server. Thus,when service 94-2 performs step 315S and makes calls, library 102-2 willnot utilize external resource agent 118-2 but, at step 415, will simplyreturn the call results (i.e. the contents of Fields 2 and 3 of Record 1from Table III) that were received by library 102-2 at step 410.

The performance of steps 405, 310S, 410 are represented in FIG. 11. Theperformance of steps 315S, 415 and 340S are represented in FIG. 12.

By the same token, when service 94-2 performs step 345S and requestsshared resources, library 102-2 will respond at step 420 by lockingshared resources 122-2, and at step 425 by returning the shared resourcesequence number (i.e. the contents of Field 4 of Record 1 from TableIII) that were received by library 102-2 at step 410 and withoututilizing sequencer 106-2.

The performance of steps 345S, 420, 425 are represented in FIG. 13.

By the same token, when service 94-2 performs step 360S and requestsreplication, library 102-2 will respond at step 430 not by actuallyperforming replication, but by returning a replication confirmation toservice 94-2 at step 380S, essentially mimicking step 400. Steps 370Sand 435 are thus performed substantially identically to steps 370 and375, respectively, such that the contents of Table IV are generatedindependently by service 94-2 and stored within shared resource 122-2.

The performance of steps 370S and 435 are represented in FIG. 14.

Similarly, steps 380S and 390S are performed in the same manner as step380 and steps 390, except that the confirmation returned at step 390S isreturned to replication agent 126-2 Instead of to client 54-1.

At this point, at the conclusion of this performance of method 300, itwill now be recognized that the results of processing message M(O₁) arenow stored in both shared resource 122-1 and shared resource 122-2 asTable IV. It can also be noted that the actual latency between theperformance of steps 310S, 315S, 340S, 345S, 360S, 370S, 380S, 390S andsteps 310, 315, 340, 345, 360, 370, 380, 390 is actually quite minimal.Any such latency can be determined by the network latency at step 365and the processing of steps 395 and 405, which can be very fast. In anyevent, system 50 can be configured so that the latency is ultimatelymuch faster than writing backup information to a hard disk, which is yetanother advantage of the present invention.

Thus, method 300 can be used to process messages to place orders to buyand sell securities using service 94-1 (and as shadowed by service94-2). Likewise method 300 can be used to cancel those orders usingservice 98-1 (and as shadowed by 98-2). Additional services can becreated and included in server 62-1 and can be readily placed ontoserver 62-2 to provide a robust failover for those services, but withoutrequiring one set of code for the service on server 62-1 while requiringanother set of code for the service on server 62-2—one set of code for aparticular service is all that is needed for both servers. Perhaps moresignificantly, from certain perspectives, is that system 50 cansubstantially guarantee the results in the event of a failover, withoutthe loss of speed that normally accompanies writing to a hard disk.

Since, in the normal state, server 62-2 maintains an up-to-date mirrorof processing performed in server 62-1, a failure of server 62-1 can bequickly recovered by having server 62-2 assume the processing tasks ofserver 62-1 where server 62-1 left-off. FIG. 15 shows a flow-chartdepicting a method 500 for managing a pair of servers where one of theservers is designated a primary server while the other server isdesignated a backup server. When implemented using system 50, at step505 it is determined if both servers are available. Step 505 isimplemented with the use of failover agents 90 and state registers 114.If yes, then step 505 advances to step 510 where system 50 operates inthe normal state as previously described in relation to method 300. Step505 and step 510 continue to cycle unless it is determined that bothservers are not available in which case the method advances to step 520.At step 520 it is determined if only the first server is available. Forexample, if failover agent 90-1 cannot establish a connection, forwhatever reason, with failover agent 90-2, then it is determined at step520 that only the first server is available and method 500 will advanceto step 530 at which point system 50 will operate in the primary-onlystate. Possible reasons for failover agent 90-1 being unable toestablish a connection with failover agent 90-2 include, but are notlimited to, server 62-2 experiencing a fatal crash, or the severing oflink 78.

If the first server is not available, then method 500 advances from step520 to step 540 where it is determined if only the second server isavailable. If not, then method 500 ends with an exception. However, ifit is determined that the second server is available, then method 500advances from step 540 to step 550. At step 550, system 50 fails over sothat further processing is performed by the second server. Next, at step560, operation continues as further processing is done in thesecondary-only state. Method 500 then cycles between step 560 and 570until both servers become available again, at which point method 500advances to step 510 and system 50 is returned to the normal state.

FIG. 16 shows an example of system 50 in the primary-only state, wherebyserver 62-1 is designated the primary server but server 62-2 is offline(or otherwise unavailable due to failure of link 78). In FIG. 16, sinceserver 62-1 is operating in the primary-only state, state register 114-1will indicate that server 62-1 is currently designated as the primaryserver and operating in the primary-only state.

FIG. 17 shows an example of system 50 in the secondary-only state,whereby server 62-2 is designated the primary server but server 62-1 isoffline. In FIG. 17, since server 62-2 is operating in the primary-onlystate, state register 114-2 will indicate that server 62-2 is currentlydesignated as the primary server and operating in the primary-onlystate.

While not shown, note that system 50 could also be configured to be thenormal state whereby server 62-2 is designated the primary server whileserver 62-1 is designated the backup server.

FIG. 18 shows as flowchart depicting a method 600 for processingmessages when only one of servers 62 is available. Method 600 would beperformed by server 62-1 in step 530 of method 500, or it would beperformed by server 62-2 in step 560 of method 500. Those skilled in theart will now appreciate that method 600 substantially reflects theoperation of the primary server in method 300. More particularly, it canbe noted that steps 310-360 and steps 370-390 of method 300 correspondto the counterparts in method 600 which bear the same numbers but arefollowed by the suffix “F”. However, step 365F of method 680 isdifferent from step 365 of method 300. Step 365F corresponds to step 430of method 300, as at step 365F library 102 will respond to the requestfor replication from service 94 (or 98) by simply mimicking theconfirmation that replication has been achieved, so that service 94 (or98) will receive such confirmation at step 380F and allow method 600 toadvance to step 390F.

FIG. 19 shows a flowchart, depicting a method 700 for failing over froma primary server to a backup server that can be used to implement step550 of method 500. Method 700 could be performed, for example, by server62-2 if failover agent 114-2 discovered that server 62-1 had failed(e.g. crashed or for whatever reason was no longer available.) Sinceclients 54 are already communicating with server 62-1, clients 54 willcontinue to interact with server 62-1, despite the fact that server 62-2will assume that server 62-1 has failed and that server 62-2 will assumethat it is the primary server. In that event, method 700 would begin atstep 710 at which point the replication agent queue would be cleared. Inthe examples discussed above, server 62-2 would continue to process alldata stored in replication agent 126-2 in accordance with step 405 (andsubsequent steps 310S, 315S, 340S, 345S, 360S, 370S, 380S, 390S, 410,415, 420, 425, 430 and 435) in order to clear out and duplicateprocessing of all messages (and associated external calls) that wasbeing processed in server 62-1 before server 62-1 failed. In the eventthat the server 62-1 fails at step 370, server 62-2 may receive aduplicate message from the client executing a recovery protocol, suchas, a gap recovery, or as another example, the type of recoverydescribed in the Applicant's co-pending application as described in U.S.Published Application US20050138461. Since the client will never receiveconfirmation from server 62-1 that the message was processed. In thisevent, server 62-2 is configured to recognize duplicate messages andsimply return the same response, without attempting to reprocess thesame message.

Next, at step 720, the replication agent would be deactivated. In thepresent example, replication agent 126-2 would be deactivated, such thatit would no longer maintain a queue of data received from server 62-1,or be configured to send messages to services 94-2 and 98-2. At step730, the external resource agent and sequencer would be activated. Inthe present example, external resource agent 118-2 would become activeso that it would be configured to make the external function calls shownin steps 325F and steps 330F of method 600. Likewise, sequence 106-2 sothat it would be configured to assign sequence numbers shown in step355F of method 600. Next, at step 740 the failover agent is set toindicate primary-only state. In the example, failover agent 114-2 is setto indicate primary-only state so that library 102-2 knows to operate inaccordance with steps 320F, 335F, 350F, 355F, and 365F of method 600.Next, at step 720, the presence of the server is announced to theclients. In the present example, server 62-2 will announce to clients 54over network 58 that server 62-2 is ready to accept and process messagesfrom clients 54. The manner in which this is done is not particularlylimited, and would substantially be the same manner in which server 62-1would have announced itself to clients 54 prior to the commencement ofmethod 300. The session protocol can perform a gap recovery sorespective sides can re-send communications that the counter party maynot have received. At this point, system 50 is in the state shown inFIG. 17, where server 62-2 is now designated as the primary server, andsystem 50 is ready to operate in the primary-only state with server 62-2as the primary server. At this point the method can return to step 560of method 500, whereby, messages from clients are received and processedin accordance with method 600.

While only specific combinations of the various features and componentsof the present invention have been discussed herein, it will be apparentto those of skill in the art that desired subsets of the disclosedfeatures and components and/or alternative combinations of thesefeatures and components can be utilized, as desired. For example, whilesystem 50 includes two servers 62-1 and 62-2 it is contemplated that anynumber of servers can be used. One server would be designated primaryserver, while any number of additional servers can be designated asbackup servers and joined together, either serially or in parallel,using suitably modified teachings herein. Such additional servers wouldhave substantially the same computing environment and structure asservers 62 disclosed herein, and in any event would have identicalservices that interact with libraries and other software elements tomake external calls (in the case of primary server) on behalf of thoseservices, or to provide replications of those external calls (in thecase of backup servers) on behalf of the mirrored copies of thoseservices.

It should also be understood that method 300 can be varied. For example,method 300 could be-configured to operate completely synchronously,whereby the primary server will only confirm to the client that amessage has been processed provided that both the primary and secondaryshared resources have been written-to with the results of the processingdone by a particular service. This can be implemented by changing method300 so step 400 is only performed once step 380S is performed.

The invention claimed is:
 1. A system for failover comprising: a primaryserver and at least one backup server; said primary server configuredto: determine a plurality of processing inputs, wherein said pluralityof processing inputs guarantees deterministic processing of saidplurality of processing inputs by said primary server and said backupserver, wherein said plurality of processing inputs includes a callresult from an external resource, said external resource resident onsaid primary server; and forward said plurality of processing inputs tosaid backup server prior to said primary server processing saidplurality of processing inputs.
 2. The system of claim 1, wherein saidexternal resource is an operating system clock.
 3. The system of claim1, wherein said plurality of processing inputs comprise a request for atime stamp.
 4. The system of claim 1, wherein said plurality ofprocessing inputs further comprise a shared resource.
 5. The system ofclaim 4, wherein said shared resource is maintained in random accessmemory of a respective server.
 6. The system of claim 1, wherein saidplurality of processing inputs further comprise a sequence numbergenerated by said primary server.
 7. The system of claim 1, wherein saidbackup server comprises a replication agent, said replication agentconfigured to communicate with a library of said primary server tofacilitate mirroring of information from said primary server to saidbackup server.
 8. The system of claim 1, wherein said backup server isconfigured to recognize a duplicate message from a client.
 9. The systemof claim 8, wherein said backup server is configured to execute arecovery protocol when said duplicate message from said client isreceived.
 10. The system of claim 9, wherein said recovery protocolcomprises sending a response, from said backup server based on mirroredinformation.
 11. The system of claim 9, wherein said recovery protocolis a gap recovery, said gap recovery results of said plurality ofprocessing inputs for sending to a respective client during a failoverto a primary-only state using backup server.
 12. The system of claim 1,wherein said primary server and said backup server operatesynchronously.
 13. A method for failover in a system comprising:determining, at a primary server, a plurality of processing inputs,wherein said plurality of processing inputs guarantees deterministicprocessing of said plurality of processing inputs by said primary serverand a backup server, wherein said plurality of processing inputsincludes a call result from an external resource, said external resourceresident on said primary server; forwarding said plurality of processinginputs from said primary server to said backup server; and processing,at said primary server, said plurality of processing inputs afterforwarding said plurality of processing inputs.
 14. The method of claim13, wherein said external resource is an operating system clock.
 15. Themethod of claim 13, wherein said plurality of processing inputs comprisea request for a time stamp.
 16. The method of claim 13, wherein saidplurality of processing inputs further comprise a shared resource. 17.The method of claim 13, wherein said plurality of processing inputsfurther comprise a sequence number generated by said primary server. 18.A non-transitory computer readable medium storing a set of programminginstructions executable on one of at least two interconnected serversvia a network, the set of programming instructions comprising:instructions for determining, at a primary server, a plurality ofprocessing inputs, wherein said plurality of processing inputsguarantees deterministic processing of said plurality of processinginputs by said primary server and a backup server, wherein saidplurality of processing inputs includes a call result from an externalresource, said external resource resident on said primary server;instructions for forwarding said plurality of processing inputs fromsaid primary server to said backup server; and instructions forprocessing, at said primary server, said plurality of processing inputsafter forwarding said plurality of processing inputs.